See those faqs:
The principle of a catalog is to record the location of media files into a database. You store a link. That link has two parts : the drive (recognized by its serial number and letter) and the full path from the root of that drive ( C:/My own pictures/...)
So, when you move your pictures to another drive, be it a network or an internal/external one, the link is broken.
There is no easy way to update the database when there is such a change. That's why the recommended solution is to do a full backup of your files on an external media, and restore it where you want, including on another drive.
Reconnecting is another possibility (in the file menu), it has been improved in PSE11, but with a large catalog like yours, I would prefer the much safer and simpler backup/restore method.
Thanks for the tip.... I first tried to backup a small catalog (ie:50 pictures) as a test. Everything worked as you described it would. After 4 nights of copying files, 1 night to create the backup and another night to restore it (The backup was approx 350GB in size and I tried it twice), PSE 11 seemed to choke on it. When the backup was restored, the files still referenced the orignal location even though I specified to restore to a new location and thus all the pictures showed as being OFFLINE. Anyway... to make a long story short: I learned of a much easier way to deal with the problem and that was as follows: On a WINDOWS 7 machine, I edited the "hosts" file (located under c:\windows\system32\drivers\etc\) and made an entry which created an alias. The file is a text file, yet has no extension and is editable using Windows NOTEPAD. The entry to make is as follows: <the IP Address of the new drive> <tab> <the name of the prior location> (ie: server3). After you make the entry, restart PSE. PSE will look for the photos at SERVER3 where they were originally stored, but the "hosts" file entry will direct it to the IP Address of the new drive. That solution took all of 2 minutes and worked like a charm!